noise_operation.h

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/*
Copyright 2025 SQUANDER Contributors

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

#pragma once

#include "density_operation.h"

namespace squander {
namespace density {

class NoiseOperation : public IDensityOperation {
public:
  bool is_unitary() const override { return false; }
};

class DepolarizingOp : public NoiseOperation {
public:
  DepolarizingOp(int qbit_num, double error_rate);

  explicit DepolarizingOp(int qbit_num);

  void apply_to_density(const double *params, int param_count,
                        DensityMatrix &rho) override;

  int get_parameter_num() const override { return is_parametric_ ? 1 : 0; }
  std::string get_name() const override { return "Depolarizing"; }

  std::unique_ptr<IDensityOperation> clone() const override;

  // Accessors
  int get_qbit_num() const { return qbit_num_; }
  double get_error_rate() const { return fixed_error_rate_; }
  bool is_parametric() const { return is_parametric_; }

private:
  int qbit_num_;
  double fixed_error_rate_;
  bool is_parametric_;

  void apply_depolarizing(DensityMatrix &rho, double error_rate);
};

class LocalDepolarizingOp : public NoiseOperation {
public:
  LocalDepolarizingOp(int target_qbit, double error_rate);
  explicit LocalDepolarizingOp(int target_qbit);

  void apply_to_density(const double *params, int param_count,
                        DensityMatrix &rho) override;

  int get_parameter_num() const override { return is_parametric_ ? 1 : 0; }
  std::string get_name() const override { return "LocalDepolarizing"; }

  std::unique_ptr<IDensityOperation> clone() const override;

  int get_target_qbit() const { return target_qbit_; }
  double get_error_rate() const { return fixed_error_rate_; }
  bool is_parametric() const { return is_parametric_; }

private:
  int target_qbit_;
  double fixed_error_rate_;
  bool is_parametric_;

  void apply_local_depolarizing(DensityMatrix &rho, double error_rate);
};

class AmplitudeDampingOp : public NoiseOperation {
public:
  AmplitudeDampingOp(int target_qbit, double gamma);

  explicit AmplitudeDampingOp(int target_qbit);

  void apply_to_density(const double *params, int param_count,
                        DensityMatrix &rho) override;

  int get_parameter_num() const override { return is_parametric_ ? 1 : 0; }
  std::string get_name() const override { return "AmplitudeDamping"; }

  std::unique_ptr<IDensityOperation> clone() const override;

  // Accessors
  int get_target_qbit() const { return target_qbit_; }
  double get_gamma() const { return fixed_gamma_; }
  bool is_parametric() const { return is_parametric_; }

private:
  int target_qbit_;
  double fixed_gamma_;
  bool is_parametric_;

  void apply_amplitude_damping(DensityMatrix &rho, double gamma);
};

class PhaseDampingOp : public NoiseOperation {
public:
  PhaseDampingOp(int target_qbit, double lambda);

  explicit PhaseDampingOp(int target_qbit);

  void apply_to_density(const double *params, int param_count,
                        DensityMatrix &rho) override;

  int get_parameter_num() const override { return is_parametric_ ? 1 : 0; }
  std::string get_name() const override { return "PhaseDamping"; }

  std::unique_ptr<IDensityOperation> clone() const override;

  // Accessors
  int get_target_qbit() const { return target_qbit_; }
  double get_lambda() const { return fixed_lambda_; }
  bool is_parametric() const { return is_parametric_; }

private:
  int target_qbit_;
  double fixed_lambda_;
  bool is_parametric_;

  void apply_phase_damping(DensityMatrix &rho, double lambda);
};

} // namespace density
} // namespace squander